Method for the production of functionalised short carbon nanotubes and functionalised short carbon nanotubes obtainable by said method

ABSTRACT

The present invention is related to a method for producing functionalised short carbon nanotubes with at least one open tip by mechanical treatment of long carbon nanotubes, wherein said long nanotubes are submitted to mechanical milling forces in the presence of a reactant able to chemically react with the nanotubes so that short carbon nanotubes comprising at least one specific chemical group are obtained.

RELATED APPLICATIONS

This is the U.S. national phase under 35 U.S.C. §371 of InternationalApplication PCT/BE01/00140, filed Aug. 21, 2001, which claims priorityto European Patent Application 00870191.4, filed Sep. 6, 2000.

FIELD OF THE INVENTION

The present invention is related to the field of carbon nanotubes. Moreprecisely, the present invention is related to the material called shortcarbon nanotubes.

STATE OF THE ART

Carbon nanotubes were first observed by Iijima in 1991 (S. Iijima,Nature 354, 56-58 (1991)) as a by-product of fullerene synthesis.Typically, the nanotubes consist of multilayers (normally 2-50) ofconcentric carbon tubes which are capped at both ends. The tubes arebuilt up of sheets of carbon atoms arranged in hexagons and pentagons,with the pentagons concentrated in areas of low radius curvature such asthe tube ends. The tubes contain a hollow core up to 50 nm acrosstypically 100-200 μm in length. Hence, single-wall tubes have been alsofound.

Their remarkable mechanical and electrical properties associated withtheir ability to be produced at large scale by arc discharge, bycatalytic decomposition of hydrocarbons, or by laser ablation forexample, explain why the carbon nanotubes are currently extensivelyinvestigated.

Nanotubes can be potentially used in various application fields such asfield emission (Q. H. Wang et al., Appl. Phys. Lett. 72, 2912-2913(1998)), electric and thermal conductivity (R. Andrews et al., Appl.Phys. Lett. 75, 1329-1331 (1999)), hydrogen storage and molecularsieves.

For applications such as hydrogen storage and molecular sieves, it hasbeen demonstrated that problems of diffusion limitation were encounteredwhen nanotubes were used (C. Liu et al., Science 286, 1127-1129 (1999);M. S. Dresselhaus et al., MRS Bulletin 24, No. 11, 45-50 (1999)). Tooverpass these problems the use of short nanotubes, ideally shorter than1 μm, with open ends has been suggested. One solution could be toproduce said short nanotubes from long carbon nanotubes. However, theproduction of such short nanotubes represents a great challenge sincerecent discussion shows that nanotubes are flexible and resistent whenstress is applied (H. Dai et al., Nature 384, 147-150 (1996); M. M. J.Treacy et al., Nature 381, 678-680 (1996); S. S. Wong et al., J. Am.Chem. Soc. 120, 8557-8558 (1998); T. Kuzumaki et al., J. Mater. Res. 13,2445-2449 (1998)).

Methods for cutting nanotubes using ultrasounds (K. L. Lu et al., Carbon34, 814-816 (1996); K. B. Shelimov et al., Chem. Phys. Lett. 282,429-434 (1998); J. Liu et al., Science 280, 1253-1256 (1998)) or STMvoltage (L. C. Venema et al., Appl. Phys. Lett. 71, 2629-2631 (1999))have been proposed. Nevertheless, these techniques are restricted tomilligram scale production. Moreover, the sample of carbon nanotubesobtained after ultrasounds treatment is relatively inhomogeneous inlength and contains only a few short carbon nanotubes, while the STMvoltage method gives short carbon nanotubes, but with closed tips.Furthermore, methods of cutting carbon nanotubes using ball milling havealso been proposed but only for the production of nanoparticles (Y. B.Li et al., Carbon 37, 493-497 (1999)), nanoporous carbon (Y. Chen etal., Appl. Phys. Lett. 74, 2782-2784 (1999)) or curved nanostructures(J. Y. Huang et al., Chem. Phys. Lett. 303, 130-134 (1999)). Inparticular, the ball milling process described by Y. B. Li et al.,Carbon 37, 493-497 (1999) uses balls and iron particles of approximately1 μm in diameter.

Moreover, for various applications, it would be of particular interestto have functionalised carbon nanotubes, and particularly shortfunctionalised carbon nanotubes. For example, this functionalisationcould allow the industrial production of composite materials through thelinkage of carbon naotubes to specific polymers. Enhancement of thephysical and mechanical properties of the carbon nanotubes could also bereached through such a functionalisation. As an example, gases storageproperties of the nanotubes could be enhanced by limiting the naturalaggregation of the nanotubes caused by Van der Waals interactions, sothat gases such as hydrogen or methane could more efficiently adsorb notonly on the inner surface of the nanotubes but also on their outersurface.

However, at the moment only few examples of chemical funtionalisationmethods have been described (J. Chen et al., Science 282, 95-98 (1998);Y. Chen et al., J. Mater. Res. 13, 2423-2431 (1998); M. A. Hamon et al.,Adv. Mater. 11, 834-840 (1999); A. Hiroki et al., J. Phys. Chem. B 103,8116-8121 (1999)) and there is still a need for methods for large scaleproduction of functionalised short carbon nanotubes.

AIMS OF THE INVENTION

The present invention aims to provide a method for producingfunctionalised short carbon nanotubes.

In particular, the present invention aims to provide a method forproducing short functionalised carbon nanotubes with open tips in gramor larger scale.

Another aim of the present invention is to provide a method forproducing short functionalised carbon nanotubes whose structure isglobally conserved comparing to the structure of long nanotubes.

Another aim of the present invention is to provide a method forproducing short functionalised carbon nanotubes with open tips at anincreased yield compared to the yields obtained until now.

The present invention also aims to provide a method for producing shortfunctionalised carbon nanotubes which can be easily and rapidlyperformed.

SUMMARY OF THE INVENTION

The present invention is related to a method for producingfunctionalised short carbon nanotubes with at least one open tip bymechanical treatment of long carbon nanotubes, wherein said longnanotubes are submitted to mechanical milling forces in the presence ofa reactant able to chemically react with nanotubes so that short carbonnanotubes comprising at least one specific chemical group are obtained.

It should be understood that the term “mechanical milling forces” refersto all mechanical forces able to mill long carbon nanotubes into shortcarbon nanotubes with at least one open tip, as opposed to chemicaltreatment and electrical treatment such as STM voltage. Examples of suchmechanical milling forces are impact forces, friction forces, shearingforces, pressure forces or cutting forces.

Preferably, the present invention is related to a method for producingfunctionalised short carbon nanotubes with at least one open tip bymechanical treatment of long carbon nanotubes, characterised in that itcomprises the step of submitting said long nanotubes to impact forces inthe presence of a reactant so that functionalised short carbon nanotubesare obtained.

Preferably, the reactant is selected from the group consisting ofliquids, solids and gases, depending on the working temperature andpressure.

Preferably, said method comprises the following steps:

-   -   making a powder containing long carbon nanotubes, the purity of        which varies from 1 to 100%;    -   introducing said powder into a ball milling apparatus containing        one or several solid particles greater than 1 mm in length,        preferably greater than 2 cm in length;    -   removing water;    -   grinding said powder with said ball milling apparatus for a        sufficient time so that a mixture containing a specific        percentage of short nanotubes with specific length is obtained,        while introducing the adequate reactant;    -   removing potential excess of reactant.

Preferably, the reactant is selected from the group consisting of air,H₂, H₂O, NH₃, R—NH₂, F₂, Cl₂, Br₂, I₂, S₈, alcohols, thiols, acids,bases, esters, peracids, peroxids, CO, COCl₂ and SOCl₂.

Preferably, the chemical or functional group introduced on the shortcarbon nanotubes produced is selected from the group consisting of SH,NH₂, NHCO, OH, COOH, F, Br, Cl, I, H, R—NH, R—O, R—S, CO, COCl and SOCl.

Preferably, the potential excess of reactant gas is removed throughheating under nitrogen atmosphere or exposition to vacuum.

Preferably, the solid particles contained in the milling apparatus areballs.

Preferably, the speed and the vertical vibration intensity of grindingare comprised within 3000-6000 vibrations/min and 0-3 mm, respectively.

Preferably, the time of grinding is comprised between 10⁻³ and 10³ h.

The grinding process may be continuous or discontinuous.

Preferably, the long carbon nanotubes are synthesised on a supportcontaining at least one metal and said long carbon nanotubes arepurified before being submitted to grinding, by dissolution of saidsupport.

Preferably, said dissolution consists in a first dissolution at atemperature comprised between 0-100° C. in a concentrated acidicsolution and in a second dissolution at a temperature comprised between100-250° C. in a concentrated basic solution, preferably a NaOHconcentrated solution. The first dissolution may be performed eitherbefore or after the second dissolution.

Preferably, the grinding is carried out in the presence of a solvent,which can be in the liquid state or in the frozen state, such as H₂O,liquid nitrogen, or an organic solvent.

Preferably, the long carbon nanotubes are submitted to at least onepre-treatment with an acid solution or a base solution and are theneventually dried.

Preferably, the long carbon nanotubes are also submitted to at least oneoxidisation pre-treatment with an oxidant in solution, or in gas phaseat temperatures above 100° C.

The long carbon nanotubes may also be submitted to at least onereduction pre-treatment with a hydrogen containing gas mixture attemperatures above 400° C.

Preferably, the method according to the invention further comprises thepurification of the functionalised short carbon nanotubes finallyobtained according to their length by classical purification methods,preferably by size exclusion chromatography.

The percentage of functionalised short nanotubes contained in themixture finally obtained according to the present invention is comprisedbetween 1 and 100%.

Moreover, the length of the functionalised short nanotubes contained inthe mixture finally obtained by the method according to the presentinvention is shorter than 50 μm, preferably shorter than 2 μm.

Preferably, the length of long carbon nanotubes to be treated by themethod according to the present invention is comprised between 1 μm and500 μm.

The long carbon nanotubes may be single-wall long carbon nanotubes ormulti-wall long carbon nanotubes or a mixture thereof.

Moreover, the present invention also relates to functionalised shortcarbon nanotubes obtainable by a method in which long nanotubes aresubmitted to mechanical milling forces in the presence of a reactant soas to allow the introduction of at least one specific chemical group onthe short carbon nanotubes produced during the milling.

The present invention also relates to functionalised short carbonnanotubes obtainable by any one of the methods mentioned hereabove.

Finally, the present invention is also related to a mixture comprisinglong nanotubes and at least 10% of functionalised short carbonnanotubes, said functionalised short carbon nanotubes having at leastone open tip and having an average length smaller than 50 μm, preferablyshorter than 2 μm.

SHORT DESCRIPTION OF THE DRAWINGS

It should be noticed that the expression <<open tip>> means that thehollow core of the nanotube is open (and accessible to small molecules)at the nanotube tip.

The word <<SWNT(s)>> is the abbreviation for single-walled carbonnanotube(s), while the word <<MWNT(s)>> is the abbreviation formulti-walled carbon nanotube(s).

SEM and TEM refer to Scanning and Transmission Electron Microscopy,respectively.

The expression <<thin MWNTs>> used hereafter refers to MWNTs having anaverage inner/outer diameter of 4/15 nm.

The expression <<thick MWNTs>> used hereafter refers to MWNTs having anaverage inner/outer diameter of 6/25 nm.

It should be noticed that, in the figures and experiments describedhereafter, the reactant used during the ball milling, if not specified,is H₂O from moist air.

FIG. 1 a represents a low magnification TEM image of thin MWNTs beforeball milling according to the present invention.

FIGS. 1 b and 1 c represent low magnification TEM images of thin MWNTsafter 12 hours of ball milling in the presence of H₂O from moist airaccording to the present invention.

FIG. 1 d represents a low magnification TEM image of thick MWNTs beforeball milling according to the present invention.

FIGS. 1 e and 1 f represent low magnification TEM images of thick MWNTsafter 120 hours of ball milling according to the present invention.

FIGS. 2 a-2 f represent the length distribution of thin MWNTs for a ballmilling time according to the present invention of 12, 10, 8, 6, 4 and 2hours for FIG. 2 a, FIG. 2 b, FIG. 2 c, FIG. 2 d, FIG. 2 e and FIG. 2 f,respectively.

FIGS. 3 a-3 e represent the length distribution of thick MWNTs for aball milling time according to the present invention of 120, 36, 16, 4and 1 hours for FIG. 3 a, FIG. 3 b, FIG. 3 c, FIG. 3 d, and FIG. 3 e,respectively.

FIGS. 4 a and 4 b represent the time evolution of carbon nanotubesaverage length for thin MWNTs and thick MWNTs, respectively, as obtainedby the present invention.

FIGS. 5 a-5 c represent X-Ray diffraction patterns of different types ofcarbon nanotubes before (curve A) and after (curve B) different ballmilling times according to the method of the present invention.

FIG. 5 a represents diffraction patterns of SWNTs before (curve A) andafter (curve B) 8 hours of ball milling.

FIG. 5 b represents diffraction patterns of thin MWNTs before (curve A)and after (curve B) 12 hours of ball milling.

FIG. 5 c represents diffraction patterns of thick MWNTs before (curve A)and after (curve B) 120 hours of ball milling.

FIG. 6 represents a high resolution TEM image of short thick MWNTs after120 hours of ball milling according to the present invention.

FIG. 7 represents the elution profile obtained by size exclusionchromatography performed on 10 mg thin MWNTs after 12 hours ball millingaccording to the present invention.

FIGS. 8 a-8 d represent TEM images of thin MWNTs after a 12 hours ballmilling according to the present invention.

FIG. 8 a represents a TEM image of thin MWNTs before separation by sizeexclusion chromatography according to the method of the presentinvention.

FIGS. 8 b-8 d represent TEM images of thin MWNTs separated by sizeexclusion chromatography according to the method of the presentinvention, the elution volume being 7.5 ml (fraction 2 of the elutionprofile), 39 ml (fraction 9 of the elution profile) and 48 ml (fraction11 of the elution profile) for FIG. 8 b, FIG. 8 c and FIG. 8 d,respectively.

FIG. 9 a represents the deconvolution of C_(1s)SXPS spectra of purifiedthin MWNTs before ball milling.

FIG. 9 b represents the deconvolution of C_(1s)SXPS spectra of purifiedthin MWNTs submitted to ball milling in the presence of NH₃.

FIG. 10 a represents the deconvolution of S_(2p)XPS spectra of thinMWNTs submitted to ball milling in the presence of H₂S.

FIG. 10 b represents the deconvolution of N_(1S)XPS spectra of thinMWNTs submitted to ball milling in the presence of NH₃.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

Production of Long Carbon Nanotubes

According to known methods, single-wall carbon nanotubes (SWNTs) andmulti-wall carbon nanotubes (MWNTS) were firstly prepared by catalyticdecomposition of hydrocarbons using a supported catalyst. The supportedcatalyst is composed of at least one metal <<supported >> on a support.The support can be for example a zeolite (such as NaY, NaX or ZSM-5), anoxide (such as MgO, Al₂O₃ or SiO₂), a mixture of oxides or a clay.

To prepare the supported catalyst, the impregnation method was performedwith a preferred concentration of 5% wt % for Co, Fe, Ni, V and Mo aloneand 2.5/2.5 wt % for Co/Fe, Co/Ni Co/V and Co/Mo. However, a total metalconcentration lower or higher than 5 wt %, composed of at least onemetal can also be used to produce SWNTs and MWNTs.

It should be noticed that the supported catalyst for MWNTs productionwas prepared according to a known process previously described by P.Piedigrosso et al., Phys. Chem. Chem. Phys. 2, 163-170 (2000); I.Willems et al., Chem. Phys. Lett. 317, 71-76 (2000); K. Hernadi et al.,Zeolites 17, 416-423 (1996).

The supported catalyst for SWNTs production was prepared according to aknown process previously described by J.-F. Colomer et al., Chem.Commun. 1343-1344 (1999) and J.-F. Colomer et al., Chem. Phys. Lett.317, 83-89 (2000).

The production of long MWNTs was carried out at 700° C. during 1 hourusing acetylene or ethylene flow of 30 ml/min and 300 ml/min of N₂ ascarrier gas.

The production of long SWNTs was carried out at 1000° C. or 1080° C.during 10 min using methane or ethylene flow of 80 ml/min and 300 ml/minof H₂ as carrier gas.

Long MWNTs finally synthesised on Co/NaY zeolite (5/95 wt %) werenanotubes with an average inner/outer diameter of 6/25 nm and a lengthof 50 μm and will be called <<long thick MWNTS>> hereafter.

Long MWNTs finally synthesised on Co/Fe/NaY zeolite (2.5/2.5/95 wt %)were nanotubes with an average inner/outer diameter of 4/15 nm and alength of 50 μm and will be called <<long thin MWNTs>> hereafter.

Long MWNTs finally synthesised on Co/Fe/Al₂O₃ zeolite (1.6/1.6/95.8 wt%) were nanotubes with an average inner/outer diameter of 5/10 nm and alength of 10 μm and will be called <<long very thin MWNTs>> hereafter.

Long SWNTs finally synthesised by catalytic decomposition of methane onCo/MgO (2.5/97.5 wt %) were nanotubes with an average diameter of 2 nmand a length of 10 μm and will be called <<long SWNTs>> hereafter.

Long MWNTs synthesized on metal(s)/support (the support being Al₂O₃,SiO₂, or zeolite) were then purified following a two step-process. Inthe first step, the metal(s) was/were dissolved in concentrated acidsolution (HCl concentrated), then the support was dissolved inconcentrated NaOH solution (40 wt %) at high temperature (100-250° C.)in order to obtain MWNTs contaminated with pyrolitic carbon. When thesupport is zeolite, an alternative first step is to dissolve the zeoliteand metal(s) in concentrated HF (38 wt %) in order to obtain MWNTscontaminated with pyrolitic carbon. In the second step, the pyroliticcarbon was eliminated according to the KMnO₄/H₂SO₄ aqueous oxidationprocedure as disclosed in K. Hernadi et al., Zeolites 17, 416-423(1996), the quantity of KMnO₄ being of 0.2 and 0.3 equivalents for longthin MWNTs and long thick MWNTs, respectively.

Long SWNTs synthesized on metal(s)/Mgo were then purified by dissolvingthe metal(s)/MgO in concentrated HCl (37 wt %) solution in order toobtain SWNTs contaminated with encapsulated metal nanoparticles.

Part I—Production of Functionalised Short Carbon Nanotubes Using H₂Ofrom Moist Air as Reactant

According to a preferred embodiment of the present invention, thestarting material is a fibrous, granular or aggregated productcontaining long thick MWNTs, long thin MWNTs, long very thin MWNTs orlong SWNTs. The SWNTs are isolated or in bundles.

The powder was introduced into a ball milling apparatus containing anagate bowl (5 cm in diameter) and available on the market under thetrademark <<Pulverisette 0>> (FRITSCH company, Germany). A ball millingwas carried out at an amplitude (vertical vibration intensity) of 3 mmand a speed of 3000 vibrations/min. The pressure was of 1 bar of moistair.

It should be noticed that the grinding in this embodiment is continuousbut a discontinuous grinding is also possible.

In this embodiment, the mechanical treatment applied to the sample inorder to break the nanotubes uses an impact force. Other types ofmechanical treatments could be used such as friction forces, shearingforces, pressure forces or cutting forces.

However, the use of an impact force produced by one ball or by severalballs, eventually of different dimensions is the preferred mechanicaltreatment. Moreover, said balls can be made of material other thanagate, as stainless steel for example.

Effect of the Ball Milling on Nanotubes

The effect of ball milling on nanotubes following the method describedhereabove was analysed from X-Ray diffraction measurements performed ona PW3710 BASED diffractometer (Philips) using CuR_(α) radiation (1.5418Å) and from transmission electron microscopy images obtained with aTecnai 10 (Philips) microscope. To prepare TEM grids, 1 mg of sample wasdispersed into 20 ml of toluene, followed by 2 minutes sonication. Thena drop was deposited on a Cu/Rh grid covered with a vinyl polymer calledformvar, and the grid was dried overnight under vacuum.

1.—Direct Observation

TEM images taken for different ball milling times are presented on FIGS.1 a-1 f for both thin and thick MWNTs. These images show that thenanotube length decreases when the ball milling time increases.

Moreover, in the particular case of thick MWNTs, nanotubes adhesion toform bundles is observed after 120 hours of ball milling, as illustratedon FIGS. 1 e and 1 f. This phenomenon is limited for mother longnanotubes and was not observed for short thin MWNTs. These differencescan be explained by differences in shape between the different types ofnanotubes. Indeed, sections of short thick MWNTs are straight, whilemother long nanotubes (see FIGS. 1 a-1 b) and short thin MWNTs (see FIG.1 c) have a curved shape, this limiting their adhesion ability.

It is also important to note, when comparing short thin and thick MWNTsas represented on FIGS. 1 a-1 f, that short thick MWNTs are individualwith continuous shape, while most of the short thin MWNTs are composedof several ca. 50-100 nm sections, these latter sections being part ofthe mother long nanotubes that have been partially cut by ball millingbut which are not disconnected (see in particular FIG. 1 a-1 c).

2.—Distribution of Carbon Nanotubes

The nanotubes length distributions obtained with thin MWNTs fordifferent ball milling times and derived from TEM images are depicted onFIGS. 2 a-2 f, while the nanotubes length distributions obtained withthick MWNTs for different ball milling times are depicted on FIGS. 3 a-3e.

These results show that the MWNTs distribution became narrow and thatthere were only short MWNTs after 10 hours of treatment for thin MWNTsand after 16 hours of treatment for thick MWNTs. It can be stated thatafter these periods all the MWNTs were broken. As can be seen on FIGS. 1e and 1 f, further treatment did not affect the global average lengthand no amorphous carbon appeared even after 120 hours of grinding.

3.—Time Evolution of Carbon Nanotubes Average Length

The evolution of the average length of short MWNTs with the grindingtime is represented on FIGS. 4 a and 4 b for thin MWNTs and thick MWNTs,respectively. On these figures, the values entitled <<experimental>>derive from the distributions of FIGS. 2 a-2 f and 3 a-3 e. Theevolution of the <<experimental>> values as a function of the ballmilling time is represented by the curves entitled <<short>>. The lattercurves do not take into account the long MWNTs length distributions,because the length of said long MWNTs can not be measured on a singleTEM picture. Therefore, it should be understood that the curves entitled<<long>> correspond to calculated values. The curves entitled <<global>>correspond to the weighted average of the preceding curves entitled<<short>> and <<long>>, considering a major contribution of the <<long>>curve during the first two hours (first period) for thin MWNTs andduring the first three hours for thick MWNTs.

As can be seen from these figures, the time evolution of MWNTs averagelength can be approximated by a decreasing exponential with aconvergence length of 0.7 μm for thin MWNTs and of 0.9 μm for thickMWNTs, that is to say an average convergence length of 0.8 μm. After 10hours for the thin MWNTs and 15 hours for the thick ones, the globalaverage length of the final MWNTs reaches its final value (0.7 μm forthin MWNTs and of 0.9 μm for thick MWNTs). This final value depends onthe thickness of mother long nanotubes originally used. Further grindingof nanotubes, up to 120 hours (FIG. 4 b), does not change the finalaverage length of the nanotubes.

4.—Structure of Nanotubes

X-Ray diffraction patterns of nanotubes before and after ball millingare shown on FIGS. 5 a-5 c curve A and curve B, respectively. Thesimilarity of these values for the MWNTs proves that the graphitizationremains almost the same for both samples, thus suggesting that thefracture is very localised and does not affect the graphene layerorganisation. Very few changes are also observed on the X-Raydiffraction patterns of the SWNTs after ball milling for 8 hours (seeFIG. 5 a).

Short MWNTs with open tips according to the invention can also beobserved on FIG. 6 which corresponds to a high resolution TEM image ofthick MWNTs after 120 hours of grinding. On the same picture, typicalnanotubes adhesion characteristics of short thick MWNTs can also beobserved.

After 10 hours for thin MWNTs and 15 hours for the thick ones, thesamples are homogeneous: all nanotubes are broken and no long nanotubesremain. Furthermore, no other forms of carbon are formed during the ballmilling procedure and the turbostratic structure of nanotubes ismaintained. The high resolution TEM image shown on FIG. 6 clearlyindicates that the nanotube structure is not damaged and that the tubeshave open tips. This latter feature is interesting for potentialapplications that would take advantage of the confinement effect in thenanotube cavity such as gas adsorption and separation or confinementlimited reactions.

5.—Complementary Analysis

It should be noticed that there is no necessity to submit nanotubes toan oxidation pre-treatment before grinding. Indeed, thin MWNTs sampleswere submitted to a pre-treatment with only HF instead of a doublepre-treatment with HF and KMnO₄ as mentioned hereabove. In theseconditions, the ball milling process leads to short MWNTs of 1 μmaverage length. Hence, the cutting rate obtained by ball milling in thiscase is lower when compared to the one obtained when the ball milling isperformed on thin MWNTs pretreated with both HF and KMnO₄ as mentionedhereabove.

It should be also noticed that other nanotubes samples such as longSWNTs (produced on Co/MgO), long very thin MWNTs (produced onCo/Fe/Al₂O₃, on Co/V/NaY or on Co/Mo/NaY), long thin MWNTs and longthick MWNTs samples produced by CVD, were successfully cut into shortnanotubes by applying the ball milling process according to the presentinvention. The nanotubes samples were either pure or contained thecatalyst and the support.

He and H₂ adsorption properties of functionalised short carbon nanotubesproduced compared to long carbon nanotubes:

He or H₂ adsorption abilities of functionalised short carbon nanotubesproduced compared to the ones of long carbon nanotubes have been studiedfor both MWNTs and SWNTs.

1. Experimental Protocol

The He and H₂ adsorption abilities of carbon nanotubes were measured bypressure swing adsorption using calibrated volumes and a precisionpressure gauge. For each pressure studied the equilibrium was reached inless than 2 minutes. The adsorption and desorption curves weresuperimposed and no histerisis was observed.

2. Case 1: MWNTs

Long very thin MWNTs were firstly synthesised according to the methoddescribed in the part entitled “Production of long carbon nanotubes”, bycatalytic decomposition of acetylene at 700° C. using a mixture of Fe/Co1.6%/1.6% supported on Al₂O₃ as catalyst. The acetylene flow was 30ml/min and N₂ was used as carrier gas at a flow of 300 ml/min. The crudevery thin MWNTs thus obtained were containing 80.2 wt % of carbon. Theyhad closed tips, were approximately 10 μm in length, had an averageinner/outer diameter of 5/10 nm and had an average number of layers of8.

Three samples were tested from these crude long MWNTs:

-   -   sample 1 containing long MWNTs as such;    -   sample 2 and sample 3 wherein 4 g fractions of long MWNTs were        submitted to ball milling according to the method of the        invention during 24 hours.

The three samples, sample 1, sample 2 and sample 3, were studied fortheir He and H₂ adsorption abilities at 77K and 295K with a workingpressure of 9 bars. Sample 1 (40 g) containing long MWNTs as such wasexposed to vacuum (10⁻⁵ Torr) at room temperature during 20 hours (step1). Sample 2 (12 g) was firstly exposed to vacuum at room temperatureduring 20 hours (step 1), then divided in 4 g fractions that weresubmitted to a 24 hours ball milling each (step 2), and then againexposed to vacuum at room temperature during 20 hours (step 3). Sample 3was submitted to the same treatment as sample 2 except that it was thenexposed to vacuum at high temperature by heating in vacuum during 5hours at 1400° C. (step 4). The crude MWTTs have lost 3 wt % on step 3and 7 wt % on step 4.

TABLE 1 summarises the obtained results (±0.001 wt % at 295 K and ±0.01wt % at 77 K) at an equilibrium pressure of 9 bars: He adsorbed H₂adsorbed (wt%) (wt%) Sample T° Crude^((a)) Pure^((b)) Crude^((a))Pure^((b)) 1 295 K 0.005 0.007 0.032 0.046 (Step 1) 77 K 0.02 0.03 0.430.61 Long MWNTs (70 wt %)^(c) 2 295 K 0.003 0.004 0.037 0.051 (Steps1-3) 77 K 0.01 0.01 0.49 0.68 Short MWNTs (72 Wt %)^(c) 3 295 K 0.0010.001 0.044 0.056 (Steps 1-4) 77 K 0.03 0.04 0.53 0.68 Short MWNTs (78wt %)^(c) ^((a))Really measured on the sample ^((b))Extrapolated to 100%of nanotubes ^((c))MWNTs content in the sample

As seen in Table 1, the He and H₂ adsorption capacities measured at 77 Kare one order of magnitude larger than the corresponding values measuredat 295 K.

The He adsorption capacities of long and short MWNTs are very low at 295K and at 77 K. The values are close to the experimental error (±0.001 wt% at 295 K and ±0.01 wt % at 77 K) and no increase of the He adsorptioncapacity was observed when passing from the long MWNTs to thefunctionalised short MWNTs.

Concerning the H₂ adsorption capacity of the crude MWNTs at 295 K (77K), values of 0.046 wt % (0.61 wt %) and 0.051 wt % (0.68 wt %) weremeasured for the long and functionalised short tubes, respectively. Itmeans that the breaking of the long MWNTs to produce short MWNTs causesa 11% (11%) increase of the adsorption capacity of the crude MWNTs. Thelatter adsorption capacity increase was characteristic of hydrogenadsorption in the central channel of the MWNTs. After the heating of theshort crude MWNTs at 1400° C. under vacuum, the H₂ adsorption capacityof the material increased to 0.056 wt % (0.68 wt %), meaning that theheat treatment caused a 10% (0%) increase of its H₂ adsorption capacity.The latter adsorption capacity increase was characteristic of hydrogenadsorption in the central channel of short MWNTs that were notaccessible before the heat treatment. The global effect of the twotreatments (ball milling and heating under vacuum) on the crude MWNTswas a hydrogen adsorption capacity increase of 22% (11%) at 295 K (77 K)

3. Case 2: SWNTs

SWNTs were synthesised by catalytic decomposition of methane at 1000° C.in presence of H₂, using Co (2.5% w/w) supported on MgO as catalyst. Theflow rate of H₂ and methane were 300 ml/min and 80 ml/min, respectively.

A concentrated HCl solution was then added to the sample in order toeliminate the support and the catalyst. The SWNTs finally contained inthe sample are about 10 μm in length and have an average diameter of 2nm. The SWNTs represent 60 wt % of the sample, the rest beingencapsulated Co nanoparticles.

The efficiency of the ball milling process on cutting andfunctionalizing SWNTs was studied by TEM, X-ray diffraction and Ramanspectroscopy. The TEM results are summarised in Table 2.

TABLE 2 Average length observed by TEM of crude single- wall carbonnanotubes as a function of the ball milling time (0.6 g of crude SWNTswas used). Ball mill time TEM analysis (h) (average SWNTs length) 0 longSWNTs (10 μm) 0.5 long + short SWNTs 1 short SWNTs (5 μm) 2 very shortSWNTs (2 μm) 3 very short SWNTs (1.5 μm) 4 pre-graphite + SWNTs (1 μm) 6pre-graphite + SWNTs (0.5 μm) 8 pre-graphite + SWNTs (0.5 μm) 24Polycrystalline graphite + SWNTs 48 Amorphous carbon 51.5 Amorphouscarbon

From the TEM observations, it was concluded that the ball millingprocess reduces the length of the SWNTs to 2 μm after 2 hours oftreatment. Further ball milling the short SWNTs reduces their lengthdown to 1 μm after 4 hours of treatment. Nevertheless, on the SWNTssamples ball milled for 4 hours or more, other forms of carbon are alsoobserved. These other forms of carbon, the formation of which isconcomitant to the destruction of the very shot SWNTs, are pre-graphite,polycrystalline graphite and amorphous carbon (Table 2).

From the X-ray diffraction analysis, it was observed that the d₁₀₀ peak(at 2θ=42.8°), characteristic of the carbon-carbon distance in carbonnanotubes and in graphite, decreases with increasing the ball-millingtime. Oppositely, the d₀₀₂ peak (at 2θ=25°), characteristic of theinterplane distance in graphite, increases with increasing theball-milling time.

On the Raman spectra, it was observed that the D band (at 1270 cm⁻¹),characteristic of disordered graphitic structures increases withincreasing the ball milling time up to 3 hours. Afterwards, it decreaseswith increasing the ball milling time and, it disappears for a ballmilling time over 50 hours. Concerning the G band (at 1597 cm⁻¹)characteristic of graphitic structures and, mainly of its shoulder (at1555 cm⁻¹) characteristic of SWNTs, they decrease with increasing theball milling time and also disappear for a ball milling time over 50hours. From the breathing modes of SWNTs (low frequency bands at 80-250cm⁻¹), it was observed that the large SWNTs are the first destroyedduring the ball milling process. After 3 hours of ball milling, thecontent of large SWNTs decreases and after 8 hours very small lowfrequency bands are observed. On the sample ball milled for 51.5 hours,none of the Raman characteristic band of SWNTs could be observed.

Three SWNTs samples, sample 4, sample 5 and sample 6 (Table 3), werestudied for their He and H₂ adsorption ability at 77 K and 295 K with aworking pressure of 9 bars. Sample 4, containing 2.8 g of long SWNTs assuch, was exposed to vacuum at room temperature during 20 hours. Sample5 and sample 6 containing 1.4 g of long SWNTs were firstly submitted toball milling according to the method of the invention during 1 hour and12 hours for sample 5 and sample 6, respectively, before being exposedto vacuum at room temperature during 20 hours. After being exposed tovacuum the SWNTs ball milled for one and 12 hours have lost 4 wt % and 6wt %, respectively.

TABLE 3 summarises the obtained results on the samples at an equilibriumpressure of 9 bars: He adsorbed H₂ adsorbed (wt %) (wt %) Crude^((a))Crude^((a)) Sample T° (±0.01) Pure^((b)) (±0.01) Pure^((b)) 4 295 K 0.020.04 0.20 0.37 Long 77 K 0.03 0.06 1.36 2.52 SWNTs (54 wt %)^(c) 5 295 K0.02 0.04 0.28 0.50 Short 77 K 0.03 0.05 1.74 3.11 SWNTs (56 wt %)^(c) 6295 K 0.01 0.02 0.32 0.56 Very short 77 K 0.02 0.04 1.86 3.26 SWNTs (57wt %)^(c) ^((a))Really measured on the sample ^((b))Extrapolated to 100%of nanotubes ^((c))SWNTs content in the sample

As seen in Table 3, the He and H₂ adsorption capacities measured at 77 Kare larger than the corresponding values measured at 295 K.

The He adsorption capacities of long, short and very short SWNTs arevery low at 295 K and at 77 K. The values are close to the experimentalerror (±0.01 wt %) and no increase of the He adsorption capacity wasobserved when passing from the long to the short or very short SWNTs.

Concerning the H₂ adsorption capacity of the crude SWNTs at 295 K (77K), values of 0.37 wt % (2.52 wt %) and 0.50 wt % (3.11 wt %) weremeasured for the long and short tubes, respectively. It means that theball milling of the long SWNTs for one hour to produce short SWNTscauses a 35% (23%) increase of the H₂ adsorption capacity of the SWNTs.The latter adsorption capacity increase was characteristic of hydrogenadsorption in the central channel of the SWNTs. After ball milling ofthe short SWNTs for 12 hours, the H₂ adsorption capacity of the materialincreased to 0.56 wt % (3.26 wt %), meaning that the last 11 hours ofball milling caused a 12% (5%) increase of its H₂ adsorption capacity.The latter adsorption capacity increase was characteristic of hydrogenadsorption in the central channel of very short SWNTs that were not yetaccessible after 1 hour of ball milling. The effect of ballmilling/functionalisation for 12 hours on the SWNTs was a hydrogenadsorption capacity increase of 51% (29%) at 295 K (77 K).

Purification of the Nano Tubes by Size Exclusion Chromatography

In order to separate functionalised short carbon nanotubes in fractionsof narrower length distribution, 10 mg of the thin MWNTs ball milled for12 hours were fractionated by size exclusion chromatography (J.-M.Bonard et al., Adv. Mater. 9, 827-831 (1997); G. S. Duesberg et al.,Appl. Phys. A 67, 117-119 (1998); G. S. Duesberg et al., Chem. Commun.435-436 (1998); G. S. Duesberg et al., Synthetic Metals 103, 2484-2485(1999)). The stationary phase was CPG 1400 Å (a Controlled-Pore Glassmaterial for column packing having a large internal surface ofcontrolled pores with free access), occupying 15 cm in length, in acolumn of 2 cm in diameter. The mobile phase was 0.25 wt % of SDS(Sodium Dodecyl Sulphate) in water.

The 10 mg of short carbon nanotubes were first dispersed in 2 ml of 1 wt% SDS in water by sonication and then introduced at the top of thecolumn conditioned with the mobile phase. Afterwards, the mobile phasewas passed throughout the column during 2 h at a rate of 27 ml/h. Afterthe death volume, 36 fractions of 1.5 ml were collected and analysed byTEM.

For the TEM analysis, a drop of the suspension was deposited on acarbonated Cu/Rh grid and the grid was dried under vacuum. Typical TEMpictures of each sample were recorded and the nanotubes lengths weremeasured manually on the pictures.

The nanotubes lengths were then used to make the length distributionhistogram of each fraction. Afterwards, the fractions were assembledgradually by 3 to generate 12 samples.

On FIG. 7 is presented the elution profile thus obtained. As seen inthis figure, the average nanotubes length decreases on increasing theelution volume. The borders of the central 50% and central 75% of eachsample gives an idea of the length distribution histogram of eachsample. The separation of the short nanotubes by size exclusionchromatography makes possible to get functionalised short carbonnanotubes of narrow length distribution (see FIG. 7). Moreover, veryshort carbon nanotubes (average length <0.1 μm; ca. fractions 712-12 onFIG. 7) of very narrow length distribution (ca. fraction 9 on FIG. 7)can also be obtained for the large elution volumes. Note that for all ofthe 12 fractions represented in FIG. 7, at least 50% of the nanotubeslength is in the range: average length ±50%.

FIG. 8 a is a typical TEM picture of the functionalised short thin MWNTsbefore their separation by size exclusion chromatography, while FIGS. 8b-8 d correspond to TEM pictures which do illustrate the narrow lengthdistribution of the functionalised short carbon nanotubes separated bysize exclusion chromatography for an elution volume of 7.5, 39 and 48ml, respectively. It should be noted that on FIG. 8 c, which correspondsto sample 9 i.e. the sample containing the smallest functionalised shortnanotubes (see FIG. 7 as reference), the ratio length/diameter goes downto 1 for some of the short nanotubes. It means that some of the shortnanotubes are smaller than 20 nm.

Part II—Production of Functionalised Short Carbon Nanotubes UsingReactants Other than H₂O from Moist Air.

Long thin MWNTs were synthesized by catalytic decomposition of acetyleneon alumina supported Co/Fe catalyst. The long thin MWNTs are purified intwo steps. First, the alumina support is dissolved by refluxing insodium hydroxide solution during two days. Secondly, the metals aredissolved by stirring in concentrated hydrochloric acid during 5 hours.The two steps were performed twice in order to remove all the catalysttraces. Finally, the long thin MWNTs are washed with water until aneutral pH is reached.

Ball-milling in specific atmosphere was used to introduce easilychemical or functional groups like thiol, amine and amide, chlorine,carbonyl, thiomethoxy, acyl chloride, hydroxyl and C-H functions, etc.on carbon nanotubes.

The functionalization of the carbon nanotubes were performed as follows:first, the carbon nanotubes were placed in a ball-mill and the systemwas either heated in nitrogen atmosphere or was exposed to vacuum inorder to remove the water. Then, the reactant gas was introduced andmaintained during the ball-milling process. Finally, the excess ofreactant gas was removed either using nitrogen stream or evacuating thesystem for 1 hour under vacuum.

It is of interest that after the ball-milling process, the apparentdensity of the functionalized carbon nanotubes increases by about oneorder of magnitude, compared to the initial long carbon nanotubes. Thisoriginates from the disappearance of the “air bubbles” existed in theweb-like nanotube sample before treatment. It is noteworthy that thisfeature is very promising in the applications of nanotubes as polymerfillers since the homogenization becomes easier.

In order to obtain more information from the breaking process twodifferent mills were used, the first is an agate mortar with a big agateball, while the second is a special metal mortar with several smallmetal balls. The MWNT samples before and after functionalization werecharacterized by X-ray Photoelectron Spectroscopy (XPS), InfraredSpectroscopy (IR), volumetric adsorption techniques and TransmissionElectron Microscopy (TEM).

The results of the volumetric adsorption measurements confirm thephysical changes. While the specific surface area of pure MWNTs isaround 250 m²/g, after the treatment (breaking and functionalization)this value increases significantly. The calculated pore radius is around20 Å after breaking, irrespective of the reactant atmosphere. Accordingto the results obtained from the volumetric adsorption measurements itfollows that the carbon nanotubes have open ends and the chemical orfunctional groups generated during the treatment leave the inner poresaccessible. Table 4 shows the specific surface areas, the pore radii,the chemical or functional groups formed during the treatment and thecharacteristic IR bands of ball-milled carbon nanotubes.

TABLE 4 BET surface area, pore radius and chemical or functional groupgenerated by ball-milling BET R_(p) Functional IR bands Sample (m²/g)(Å) Groups^(a) (cm⁻¹) MWNTs 254 67 (—OH, —COOH)^(b) — MWNTs broken inH₂O 291 20 —OH — MWNTs broken in 288 20 —SH 791 H₂S MWNTs broken in NH₃276 20 —NH₂, —CONH₂ 1490 MWNTs broken in 192 20 —Cl — Cl₂ MWNTS brokenin CO 283 20 >C═O 1675 MWNTs broken in 294 20 —SCH₃ 615 CH₃SH MWNTsbroken in 278 20 —COCl 1785 COCl₂ MWNTs broken in H₂ 295 20 —H — SWNTs757 — (—OH, —COOH)^(b) — SWNTs broken in 1500 — —OH — H₂O ^(a)Only themost abundant functions are represented. ^(b)The —OH and —COOHfunctions, measured by titration, were introduced during thepurification of the nanotubes.

Deconvoluting the C_(1s) XPS spectra of purified MWNTs (FIG. 9 a) and ofpurified MWNTs functionalised with NH₃ (FIG. 9 b), five peaks areobtained. The first one is observed at 284.5 (±0.1) eV and is due tosp²-hybridized carbon atoms and carbon atoms bonded to hydrogen atoms.The peaks for sp³-hybridized carbon atoms are centered at 285.1 (±0.1)eV. The peaks at 286.1 (±0.2) eV, 287.4 (±0.2) eV and 289.0 (±0.1) eVrepresent, the carbon atoms bonded to one oxygen atom by a single bond(e.g., alcohol, ether), by a double bond (e.g. ketone, aldehyde, amide)and to two oxygen atoms (e.g. ester, carboxylic acid), respectively. Thepeak at 291.0 (±0.1) eV is characteristic of the shake-up of thesp²-hybridized carbon atoms.

The S_(2p) XPS spectra of MWNTs which have been treated with H₂S showone component at 163.6 (±0.2) eV (FIG. 10 a). This value corresponds tofree mercaptans.

The deconvolution of N_(1s) XPS spectra of ammonia treated MWNTs showstwo species: the first at 399.0 eV and the second at 400.5 eV (FIG. 10b). The first peak is attributed to amine functional groups and thesecond is due to the presence of amide.

From the experimental results, a simple mechano-chemical way offunctionalization can be assumed. If two different ball-milling systemsare compared it seems that the efficiency of breaking depends on thegeometry of the mill and the duration of the treatment. It seems thatcleavage starts not only at places of defects, but also the mechanicalstress induces the formation of defects and, finally, the cleavage ofthe tubes. Surprisingly the cleavage of the C-C bonds takes place in thepresence of NH₃, Cl₂, H₂S, H₂O, so that new bonds between the carbonnanotubes and the reactant are formed. Certainly, the efficiency of thisreaction strongly depends on the reactant, albeit in our case the solidmaterial obtained after treatments contained functional groups in ratherhigh quantity.

In conclusion, the ball-milling induced functionalisation of MWNTs underreactive atmospheres allows the production of short carbon nanotubescontaining different chemical functions. The process can be carried outon large scale (up to 50 g per reaction actually) resulting in highamount of functionalized short nanotubes. Introduction of amine andamide functional groups using ammonia as well as the introduction ofthiol using hydrogen sulfide was confirmed by the XPS results. Otherchemical or functional groups can also be easily introduced by thistechnique. Moreover, these preliminary results, summarised on Table 4,show that the technique can be applied not only for multi-walled butalso for single-walled nanotubes.

1. A method for producing functionalised short carbon nanotubes with atleast one open tip comprising mechanically treating long carbonnanotubes in a powder form, wherein a reactant gas able to chemicallyreact with said long nanotubes is chosen and said long carbon nanotubesin a powder form are submitted to mechanical milling forces in thepresence of said reactant gas so that short carbon nanotubes, comprisingat least one specific chemical group, are obtained, and wherein saidreactant gas is not air.
 2. The method according to claim 1, wherein thetreating comprises: making a powder containing long carbon nanotubes;introducing said powder into a ball milling apparatus containing one orseveral solid particles of at least 1 mm in length; grinding said powderwith said ball milling apparatus for a sufficient time so that a mixturecontaining a specific percentage of short nanotubes with specific lengthis obtained, while introducing the adequate reactant gas, said reactantgas being not air; and removing any excess of the reactant gas.
 3. Themethod according to claim 1, wherein the reactant gas is selected fromthe group consisting of: H₂, H₂O, NH₃, R—NH₂, F₂, Cl₂, Br₂, I₂, S₈,alcohols, thiols, acids, bases, esters, peracids, peroxids, CO, COCl₂and SOCl₂.
 4. The method according to claim 1, wherein the chemicalgroup introduced on the short carbon nanotubes produced is selected fromthe group consisting of: SH, NH₂, NHCO, OH, COOH, F, Br, Cl, I, R—NH,R—O, R—S, CO, COCl and SoCl.
 5. The method according to claim 2, whereinany excess reactant gas in the ball milling apparatus is removed usingnitrogen stream or evacuation to vacuum.
 6. The method according toclaim 2, wherein the time of grinding is between 10⁻³ and 10³ hours. 7.The method according to claim 2, wherein said grinding is continuous. 8.The method according to claim 2, wherein said grinding is discontinuous.9. The method according to claim 1, further comprising purifying theshort carbon nanotubes according to their length by size exclusionchromatography.
 10. The method according to claim 1, wherein thepercentage of short nanotubes contained in the mixture is between about1% and about 100%.
 11. The method according to claim 1, wherein thelength of the short nanotubes contained in the mixture is shorter than50 μm.
 12. The method according to claim 1, wherein length of the longcarbon nanotubes is between 1 μm and 500 μm.
 13. The method according toclaim 1, wherein the long carbon nanotubes are single-wall long carbonnanotubes or multi-wall long carbon nanotubes or a mixture thereof. 14.The method according to claim 2, wherein the length of the shortnanotubes contained in the mixture is shorter than about 2 μm.